The present invention relates to an ignition unit for an internal combustion engine for applying high voltage for ignition onto a spark plug and causing the spark plug to perform a spark discharge.
It is known that an amount of spark energy required for obtaining a normal combustion of mixed gas within an internal combustion engine is dependent on operating conditions of the internal combustion engine. The spark energy can be expressed by an amount of discharge current (secondary current) that is made to flow through spark discharge and by duration of spark discharge.
When the engine is in a low-revolution and low-load condition at the time of idling driving or the like, combustion of mixed gas is progressed in at a very sluggish speed due to a small loading amount of mixed gas to a combustion chamber and a slow flow velocity of turbulent flow (swirl flow or tumble flow) of mixed gas. For achieving stable combustion in such a low-revolution and low-load condition, it is thus necessary to increase the spark energy for supporting growth of a flame core to thereby support combustion of mixed gas. On the other hand, in a high-revolution and high-load condition, combustion is progressed at a high speed owing to a large loading amount of mixed gas to the combustion chamber and a high density of mixed gas so that a relatively small spark energy suffices.
A conventional ignition unit for an internal combustion engine was therefore arranged to be capable of supplying maximum spark energy required for various operating conditions of the internal combustion engine so as to prevent shortage of spark energy.
However, the supply of spark energy will be excess when the conventional internal combustion engine may be operated with smaller spark energy than the maximum required spark energy. This excess supply of spark energy will neither contribute to ignition of the mixed gas nor will it cause an excess increase in electrode temperature of the spark plug to thereby lead to faster exhaustion of the electrodes.
It is another drawback that the flow velocity of turbulent flow of the mixed gas becomes stronger (faster) the more the internal combustion engine approaches its high-rotation and high-load operating condition to cause repeated phenomena (so-called multiple discharge) wherein sparks are flown to a downstream side in the latter half of spark discharge in which the spark energy is decreasing until the spark discharge is finally blown off and repeatedly generated. In the presence of such phenomena, fusing or spattering of electrode materials of the spark plug is promoted owing to aggregation of sparks on the downstream side and owing to abrupt increases in electrode temperature to cause so-called irregular exhaustion with downstream side electrodes being particularly exhausted and the life of the spark plug is shortened in vain.
On the other hand, a so-called full-transistor type ignition unit is becoming common in these years for use as an ignition unit for an internal combustion unit that employs a switching element comprised of a semiconductor element such as a power transistor or the like as a means for switching between an energized/deenergized (interrupted) condition of a primary winding of the ignition coil for applying high voltage for ignition on the spark plug. In such an ignition unit of full-transistor type, time for energizing the primary coil of the ignition coil can be easily controlled by adjusting a time for driving the switching element (ON time). It is therefore possible in such a type of ignition unit for an internal combustion engine to control the spark energy to be of an amount required for combustion of mixed gas by controlling the time for energizing the primary wiring of the ignition coil in accordance with operating conditions of the internal combustion engine.
However, in performing control of the time for energizing the primary winding through the ignition coil before generation of spark discharge through the spark plug, an amount of energy stored in the ignition coil through the energizing will become small when the time for energizing is short, and the high voltage for ignition generated in a second wiring by interrupting energizing will accordingly become small. Consequently, when the time for energizing the primary wiring is set to be short for the purpose of, for instance, reducing the amount of spark energy at the time the internal combustion engine is in a high-rotation and high-load condition, the high voltage for ignition generated in the second wiring by energizing/deenergizing the first wiring of the ignition coil will become small so that it is impossible to obtain a high voltage for ignition suitable for a high-rotation and high-load operating condition wherein voltage required for ignition of the spark plug is high and may lead to misfire.
It is an object of the present invention to provide an ignition unit for an internal combustion engine wherein spark energy is minimized without controlling time of energizing a primary wiring of an ignition coil prior to generation of spark discharge, and to achieve long-life of a spark plug by restricting multiple discharge that is apt to be generated under a high-rotation and high-load operating condition.
According to one aspect of the present invention, the ignition unit for an internal combustion engine comprises an ignition coil including a primary wiring connected to a power source unit and a second wiring forming a closed loop together with a spark plug equipped in the internal combustion engine; a spark discharge generating means for energizing current from the power source unit to the primary wiring of the ignition coil synchronously with the rotation of the internal combustion engine and for generating high voltage for ignition in the second wiring by interrupting the energizing current to thereby make the spark plug perform spark discharge; a primary wiring short-circuiting means for short-circuiting both ends of the primary wiring of the ignition coil in correspondence to instructions from an instruction means; a spark discharge duration calculating means for calculating a spark discharge duration required for combusting mixed gas through spark discharge of the spark plug based on an operating condition of the internal combustion engine; and a spark discharge interrupting means for forcibly interrupting spark discharge of the spark plug by short-circuiting both ends of the primary wiring of the ignition coil by actuating the primary wiring short-circuiting means in accordance with the spark discharge duration calculated by the spark discharge duration calculating means.
By short-circuiting both ends of the primary wiring through the primary wiring short-circuiting means when spark discharge is being generated, current starts to flow through a closed loop formed by the primary wiring and the primary wiring short-circuiting means through a magnetic flux remaining in the ignition coil. This current being gradually increased and voltage which is of opposite polarity to that of the high voltage for ignition that had been generated in the secondary wiring when spark was generated, being induced on the secondary wiring, through magnetic flux remaining in the iron core of the ignition coil, spark discharge at the spark plug is forcibly interrupted or extinguished.
Spark discharge will not be immediately interrupted upon short-circuiting both ends of the primary wiring, but the spark discharge is only interrupted when a primary current has increased to a level for inverting the polarity of induced voltage generated in the secondary wiring after short-circuiting both ends of the primary wiring. It is therefore necessary to perform short-circuiting of both ends of the primary wiring through the switching element prior to an interrupting timing for the spark discharge, wherein the time between short-circuiting both ends of the primary wiring and interrupting the spark discharge becomes longer the more magnetic flux is remaining in the ignition coil and shorter the less magnetic flux is remaining. However, since the amount of magnetic flux remaining in the ignition coil is determined by the spark discharge duration, it will be possible to perform interruption of spark discharge at a proper timing by setting the timing for short-circuiting both ends of the primary wiring in accordance with the spark discharge duration.
In other words, the present invention does not perform control time for energizing the primary wiring prior to the spark discharge generation based on the operating condition of the internal combusting engine for limiting excess supply of spark energy, but controls the spark discharge duration by forcible interrupting spark discharge so that it is enabled to set the time for energizing the primary wiring prior to the generation of spark discharge to be sufficiently longer. Accordingly, high voltage for ignition generated in the second wiring can be applied to the spark plug by an amount that is large enough for performing reliable ignition in various operational conditions of the internal combustion engine, and occurrence of misfire can be limited.
By arranging the spark discharge duration to be calculated based on the operating condition of the internal combustion engine, excess supply of spark energy can be limited by calculating the spark discharge duration to be short in an operating condition requiring only a small amount of spark energy (e.g. when performing high-rotation and high-load). On the other hand, it is enabled to reliably combust mixed gas by calculating the spark discharge duration to be long in an operating condition in which mixing gas is hard to be ignited, for instance, when the internal combustion engine in a low-rotation and low-load condition.
Further, by setting the spark discharge duration to be short in a high-rotation and high-load condition in the above-described manner, it is possible to effectively limit generation of multiple discharge in a condition in which turbulent flow of mixed gas is strong, such as in the high-rotation and high-load condition. In other words, the spark discharge duration is set to be short in a high-rotation and high-load operating condition since ignition properties to the mixed gas are favorable and ignition can be performed also with small spark energy, while spark discharge is interrupted prior to the generation of multiple discharge that is apt to be generated in the latter half of spark discharge to thereby limit multiple discharge.
The present invention is particularly effective when being applied to an internal combustion engine such as a lean-burn engine performing combustion at an air-fuel ratio of not less than 20. In general, the flow velocity of turbulent flow of mixed gas is made strong in an internal combustion engine performing combustion at a lean airfuel ratio because it is impossible to achieve stable ignition properties unless the lean fuel is made to be a uniformly dispersed mixed gas before spark discharge is started. This leads to the fact that multiple discharge is apt to occur in the latter half of spark discharge in which the spark energy is degraded and exhaustion of electrodes (irregular exhaustion) of the spark plug is apt to be promoted. On the other hand, the ignition properties to the mixed gas will be favorable even when the spark energy is small when the internal combustion engine is in the high-rotation and high-load operating condition even though this engine be one combusting at lean air-fuel rates. Thus, by employing the ignition unit for an internal combustion engine of the present invention for reducing the spark discharge duration in the high-rotation and high-load condition and interrupting spark discharge prior to the generation of multiple discharge that is apt to be generated in the latter half of spark discharge, it is possible to secure favorable ignition properties while limiting generation of multiple discharge.
For forcibly interrupting spark discharge, it is possible to employ an arrangement wherein current is made again to flow through the primary wiring by using the external power source through which current was applied on the primary wiring for generating spark discharge and the switching means. According to this style, the time between short-circuiting of the switching means and interruption of the spark discharge can be decreased since current is forcibly applied to the primary wiring through the external power source, and it is possible to set the spark discharge duration to be even shorter. However, since current is supplied by the external power source, the amount of current energized to the primary wiring tends to be large and a heating value of a semiconductor element will accordingly become larger when using the semiconductor element as the switching means. It will thus be necessary to employ an expensive semiconductor element that exhibits superior durability to stand heat, and thus leads to a problem of increased costs. While it is further possible to provide a plurality of semiconductor elements of low cost to restrict heating values of these semiconductor elements by dispersing the energized current, such a measure will lead to increases in the number of parts to make the controlling process complicated.
In contrast thereto, since the ignition unit for an internal combustion engine according to the present invention is not arranged to interrupt sparks through re-energizing from an external power source, it is possible to make the amount of current to be energized to the primary wiring small at the time of interrupting sparks, and it is not necessary to employ an expensive semiconductor element of superior durability when using a semiconductor element as the primary wiring short-circuiting means. Further, since the primary wiring short-circuiting means merely needs to perform short-circuiting of both ends of the primary wiring, the means may be realized, for instance, by using a single switching element to thereby simplify controlling processes.
It is therefore enabled by the present invention to realize an ignition unit for an internal combustion engine for performing interruption of sparks as a simple arrangement employing a small number of parts and using parts made of low-cost materials.
On the other hand, since the primary wiring short-circuiting means merely needs to perform short-circuiting of both ends of the primary wiring as described above, it is also possible to employ a mechanical relay switch or similar besides a semiconductor switching element such as a transistor or a bi-directional three-terminal thyristor. However, due to the fear that spark discharge might be repeatedly generated upon abruptly interrupting the primary current when both ends of the primary wiring are released before the magnetic flux remaining in the ignition coil is completely consumed, it is necessary to keep on short-circuiting both ends of the primary wiring until no current is flown in the primary wiring. In addition thereto, since the amount of magnetic flux remaining in the ignition coil is dependent on the spark discharge duration as explained above, the timing for releasing both ends of the primary wiring needs to be set based on the spark discharge duration.
It is therefore preferable to arrange the primary wiring short-circuiting means by a switching element permitting energizing only in a direction for consuming the magnetic flux stored in the ignition coil in accordance with instructions from an instruction means for short-circuiting both ends of the primary wiring of the ignition coil, and releasing both ends of the primary wring thereafter in the absence of current flowing in the permitted direction.
In other words, after the primary wiring short-circuiting means starts short-circuiting of the primary wiring upon receipt of instructions from an instruction means, current is terminated from flowing in the primary wiring irrespective of the instructions so that both ends of the primary wiring are automatically released. It can thereby be eliminated for the necessity of performing controlling processes for setting a timing for releasing the primary wiring based on the spark discharge duration and performing controlling processes for releasing the primary wiring short-circuiting means whereby controlling processes of the primary wiring short-circuiting means can be simplified.
Moreover, since voltage is applied to make current flow in a direction opposite to the permitted direction also in case energizing of the primary wiring is started prior to the spark discharge without complete consumption of magnetic flux remaining in the ignition coil, the primary wiring short-circuiting means will release both ends of the primary wiring. It can therefore be prevented that both ends of the primary wiring are erroneously short-circuited by the primary wiring short-circuiting means at the time of energizing the primary wiring prior to the spark discharge to thereby interfere storage of spark energy by the ignition coil.
It should be noted that methods for short-circuiting both ends of the primary wiring may be, for instance, a method for short-circuiting both ends of the primary wiring in a circuit comprised by a pup transistor (Tr1) and a npn transistor (Tr2) or a method for short-circuiting both ends of the primary wiring by means of a thyristor with an external instruction signal being input to a gate thereof.
For instance, when short-circuiting both ends of the primary wiring in a circuit comprised of transistors, a circuit shall be employed that is arranged by connecting a base of Tr1 to a collector of Tr2 and a collector of Tr1 to a base of Tr2, wherein the emitters of Tr1 and Tr2 are respectively connected to the primary wiring. This circuit is further arranged in that when the external instruction signal input to the base of Tr2 is of high level, Tr2 will be in an ON condition while Tr1 will be in an ON condition as well through base current flowing to Tr1. Since base current will accordingly be supplied further to Tr2 through the collector of Tr1, Tr1 and Tr2 will maintain their ON conditions as long as base current is supplied to Tr2 even though the external instruction signal is changed to low level to thereby make the primary current flow through the primary wiring. Thereafter, when the primary current becomes small and Tr2 comes to an OFF condition, Tr1 will accordingly come to an OFF condition to release both ends of the primary wiring. With this arrangement, it can be eliminated for the necessity of setting timings for releasing both ends of the primary wiring and performing controlling processes for releasing the primary wiring short-circuiting means. It should be noted that current is made to flow to the primary wiring by permitting energizing only in a direction in which current is flown in from the emitter of Tr1 and flown out from the emitter of Tr2.
For short-circuiting both ends of the primary wiring by using a thyristor, the thyristor shall be connected in a parallel manner to the primary wiring as illustrated in the embodiment to be described later. According to the circuit of the embodiment to be described later, energizing is permitted only in a direction for energizing the thyristor when the external instruction signal input to the gate comes to a high level and the thyristor is switched to an ON condition so that both ends of the primary wiring are short-circuited for making the primary current flow. The ON condition of the thyristor is continued unless the primary current is kept on being energized even though the external instruction signal that is input to the gate comes to a low level. When no primary current is energized anymore, the thyristor comes to an OFF condition to release both ends of the primary wiring. Such a method employing a thyristor may be arranged by using a single semiconductor element (thyristor) and does not require utilization of a plurality of semiconductor elements like a circuit arranged of a transistor, and it is possible to advantageously realize the circuit through a simple arrangement of low cost.
Therefore, by employing the above-described arrangement, it is possible to simplify controlling processes for spark interruption and to prevent malfunctions interfering storage of spark energy by the ignition coil, and it is possible to realize an ignition unit for an internal combustion engine of low cost and of high reliability.
It is a second noteworthy point of the present invention that a gate terminal of the switching element is further connected to a driving signal output means for outputting a driving signal of a higher potential than a potential of a positive terminal of the power source unit upon receipt of an instruction signal output from the spark discharge interrupting means in accordance with the spark discharge duration.
For driving the switching element that is connected in a parallel manner to both ends of the above-described primary wiring, it is necessary to input a specified amount of current to the gate terminal sufficient for driving the switching element and to input a signal to the gate terminal that is of a higher potential than a potential of a cathode terminal that is connected to the positive terminal of the power source unit. For inputting a driving signal to the gate terminal of the switching element of a higher potential than the potential of the positive terminal of the power source unit, it may be possible to incorporate, for instance, a step-up unit (DC/DC converter etc.) for outputting the above driving signals within an engine control unit (so-called ECU) for comprehensively controlling the internal combustion engine including signal control (ignition control) for energizing/deenergizing a main control switching means. However, in realizing the ignition unit for an internal combustion engine of the present invention by modifying a conventionally used engine control unit, the number of items of which arrangements are to be on the engine control unit side (modifying items) will be increased. Moreover, when making the engine control unit incorporate the step-up unit such as a DC/DC converter or the like, a step-up transformer that is generally provided in a step-up circuit will become a noise source to induce malfunctions in a microcomputer or the like comprising the engine control unit, such that the control of the internal combustion engine through the engine control unit may become unstable.
However, according to the other aspect of the present invention, it is not necessary to provide a step-up unit on the engine control unit side since the driving signal output means is arranged to output a driving signal, which is of higher potential than the potential of a positive terminal of the power source unit, to the gate terminal of the switching element upon receipt of an instruction signal corresponding to the spark discharge duration. In other words, the switching element is driven by using an arrangement for outputting a potential to the gate terminal of the switching element, that is higher than that of the positive terminal (cathode terminal) of the power source unit for short-circuiting both ends of the primary wiring, the arrangement being comprised by the driving signal output means provided separate from the engine control unit and at the ignition unit itself. Consequently, control of the internal combustion engine through the engine control unit will not become instable also when driving the switching element.
The driving signal output means for outputting a driving signal, which is of higher potential than the potential of a positive terminal of the power source unit, to the gate terminal of the switching element may be preferably comprised of a capacitor which one terminal (hereinafter referred to as xe2x80x9cfirst terminalxe2x80x9d) is connected to the gate terminal of the switching element and a charge/discharge control means connected to the other terminal of the capacitor (hereinafter referred to as xe2x80x9csecond terminalxe2x80x9d) for controlling charge/discharge of the capacitor. A driving signal of higher potential than the potential of the positive terminal of the power source unit is output to the gate terminal of the switching element by the action of the charge/discharge control means for charging the capacitor by setting the potential of the second terminal of the capacitor to a potential of the negative terminal side of the power source unit while the capacitor is discharged such that the potential of the first terminal comes to a higher potential than the potential of the positive terminal of the power source unit based on an instruction signal.
More particularly, the charge/discharge control means sets the potential of the second terminal of the capacitor to the potential of the negative terminal side of the power source based on an instruction signal from the spark discharge interrupting means to thereby charge the capacitor through power source supply from the power source unit. In this manner, the capacitor will be charged until a voltage thereof becomes equal to the power source voltage of the power source unit. At this time, the capacitor is charged such that the first terminal becomes a high potential and the second terminal a low potential. The charge/discharge control means then makes the capacitor discharge by setting the potential of the second terminal to a potential of the positive terminal side of the power source unit (such that the potential of the first terminal becomes a higher potential than the potential of the positive terminal of the power-source unit) based on an instruction signal from the spark discharge interrupting means. At this time, the potential of the second terminal of the capacitor will become substantially equal to the potential of the positive terminal of the power source unit the moment discharge of the capacitor is started, and the potential of the first terminal will become a potential corresponding to the potential of the power source unit increment by the voltage of the capacitor at the time of charge. Thus, when discharge is started by the capacitor, the potential of the first terminal will at least be a higher potential than the potential of the positive terminal of the power source while discharge current of the capacitor is made to flow into the gate terminal of the switching element so that the switching element is driven.
Therefore, according to the above arrangement, it is possible to arrange a driving signal output means capable of driving a switching element that is connected in a parallel manner to both ends of the primary wiring without providing an expensive step-up unit, and to provide an ignition unit for an internal combustion engine of reduced costs.
It is preferable that a current restricting means for restricting an amount of discharge current when the capacitor performs discharge be serially connected to the capacitor. By the provision of the current restricting means, the amount of discharge current flowing from the capacitor to the gate terminal of the switching means can be restricted when discharge of the capacitor is performed through the charge/discharge control means. It can thus be prevented that excess discharge current (rush current) be flown into the gate terminal of the switching element when forcibly interrupting spark discharge and to effectively prevent damages of the switching element.
It is further preferable that a noise eliminating means be provided between a connecting point of the gate terminal of the switching element and the first terminal of the capacitor and the power source unit for preventing entrance of noise to the gate terminal of the switching element.
By the provision of the noise eliminating means, the switching element will not come to a short-circuited condition at an improper timing, and it is thus possible to prevent abnormal generation of spark discharge and an improper interrupting timing for spark discharge. Accordingly, the internal combustion engine can be stably operated even when employing an arrangement wherein the switching element, which is connected to both ends of the primary wiring in a parallel manner, is driven by using the driving signal output means comprising the above-described capacitor and the charge/discharge control means.
The ignition unit for an internal combustion engine of the present invention works better when being employed with a gas engine using a gaseous fuel as fuel.
Since gaseous fuel exhibits higher insulating properties in contrast to gasoline which is a liquid fuel, the spark discharge voltage will be relatively higher. It is therefore necessary to set a maximum secondary voltage generating performance for the ignition coil suitable for use with a gas engine using a gaseous fuel to be higher than that of one used with a gasoline engine (for instance, when the maximum secondary voltage of the ignition coil suitable for use with a gasoline engine is not less than 30 kV, that of a ignition coil suitable for use with an gas engine is set to be not less than 40 kV). It is thus required to design the ignition coil to increase a primary/secondary turns ratio as well as a wiring number of the primary wiring and the second wiring or to increase the primary current value for performing interruption.
However, while the maximum secondary voltage generating performance can be increased employing the above-described design for the ignition coil, it will simultaneously cause a drawback of increasing the spark energy. This is due to influences of a reciprocal relationship between the spark discharge duration and a maximum secondary current wherein a peak value of the secondary current increases when designing the spark discharge duration to be short (designing the ignition coil to decrease the primary/secondary turns ratio) so that exhaustion of the electrode of the spark plug is promoted through increase in energy density. Further, when designing the secondary current value to be small (designing the ignition coil to increase the primary/secondary turns ratio), it is possible to decrease the peak value of the secondary current while increasing the spark discharge duration instead which, in turn, affects the exhaustion of the electrode of the spark plug. In other words, the amount of unnecessary supply of spark energy to the spark plug is considered to be larger when using a gas engine rather than a gasoline engine so as to further shorten the life of the spark plug.
Therefore, by applying the ignition unit for an internal combustion engine of the present invention to the above-described gas engine using gaseous fuel, it is possible to effectively prevent excess supply of spark energy and further to improve the maximum secondary voltage (high voltage for ignition) generating performance, and thus to work best for exhibiting the effect of achieving a long life of the spark plug.
The ignition unit for an internal combustion engine of the present invention becomes effective when being applied particularly to a stationary gas engine among gas engines. Since fuel economy is an important factor in view of performance of a stationary gas engine, leaning is promoted for achieving sparing of fuel. It is therefore necessary for the stationary gas engine to make the flow velocity of turbulent flow of mixed gas stronger for effectively combusting at a lean air-fuel ratio so that multiple discharge tends to be generated between electrodes of the spark plug. Thus, by applying the ignition unit for an internal combustion engine of the present invention to a stationary gas engine, it is enabled to restrict generation of multiple discharge and to restrict exhaustion of electrodes (irregular exhaustion) of the spark plug.